Transfer case with four-wheel underdrive operating mode

ABSTRACT

A transfer case for a four-wheel drive vehicle includes a center differential which provides a direct output in two-wheel drive and a slightly reduced speed output in four-wheel drive. In two-wheel drive, the outputs of the center differential are locked together by a clutch collar and drive torque is provided to the primary drive line only. In four-wheel drive, the clutch collar unlocks the differential and couples the output from a sun gear to a chain drive sprocket in the secondary drive line. The center differential is driven through the carrier and operates as an open differential. The ring (outer) gear drives the primary drive line and the sun (center) gear drives the secondary drive line through cooperating chain sprockets and a chain. The chain drive sprockets are of unequal size and effect a speed reduction between the drive and driven sprockets. This speed reduction is nominally the same as the speed reduction from the input to the primary output achieved through the center differential. Thus, in four-wheel drive mode, the drive (speed reduction) ratio through the drive lines is raised, improving the acceleration and pulling power of the vehicle. In two-wheel drive, more favorable fuel economy is provided with the lower drive ratio.

CROSS REFERENCE TO CO-PENDING APPLICATION

This application is a divisional application of Ser. No. 09/191,186,filed Nov. 13, 1998 now U.S. Pat. No. 5,984,821, granted Nov. 16, 1999.

BACKGROUND OF THE INVENTION

The invention relates generally to transfer cases for four-wheel drivevehicles and more specifically to a transfer case having a centerdifferential which is configured to selectively provide a four-wheelunderdrive operating mode.

The obvious benefits of improved traction and vehicle control achievedby four-wheel drive systems in adverse driving conditions such as snow,freezing rain, ice and even water has been known and appreciated byvehicle designers for many years. A not so obvious benefit relates tothe use of four-wheel drive when towing a trailer carrying a boat,snowmobiles and the like. Here, too, the improved traction providesimproved driver control and stability in adverse driving conditions.When trailer towing is viewed as a specific operational mode, additionalfeatures and requirements may be added to the list of vehicle designcriteria. For example, when the vehicle weight is augmented by severalhundred or several thousand pounds, a slightly higher overall driveratio provides improved torque and thus improved acceleration.

In contrast, when the vehicle is utilized in two-wheel drive thepresumption exists that neither the enhanced traction nor improvedacceleration would be beneficial or necessary inasmuch as either thevehicle is not towing a trailer, the road conditions are good or both.

The present invention is directed a transfer case having a centerdifferential which provides direct two-wheel drive and an underdrivefour-wheel drive operating mode which improves vehicle performance.

SUMMARY OF THE INVENTION

A transfer case for a four-wheel drive vehicle includes a center,planetary gear differential which provides a direct output in two-wheeldrive and a slightly reduced speed output in four-wheel drive. Intwo-wheel drive, the outputs of the center differential are lockedtogether by a clutch collar and drive torque is provided to the primarydrive line only. In four-wheel drive, the clutch collar unlocks thedifferential and couples the output from a sun gear to a chain drivesprocket in the secondary drive line. The center differential is driventhrough the carrier, operates as an open differential and is preferablyconfigured with a 65% primary (rear), 35% secondary (front) drive linetorque split. The ring (outer) gear drives the primary drive line andthe sun (center) gear drives the secondary drive line throughcooperating chain sprockets and a chain. The chain sprockets are ofunequal size and effect a speed reduction between the drive and drivensprockets. This speed reduction is nominally the same as the speedreduction from the input to the primary output achieved through thecenter differential. Thus, in four-wheel drive mode, the drive ratiothrough the drive lines is raised and the speed reduced, improving theacceleration and pulling power of the vehicle. In two wheel drive, morefavorable fuel economy is provided with the lower (direct) drive ratio.With the open center differential, skid control and torque distributionmay be achieved through the vehicle's antilock or anti-skid brakingsystem.

It is thus an object of the present invention to provide a transfer casehaving both direct drive and underdrive modes of operation.

It is a further object of the present invention to provide a transfercase having a center differential which may be locked out to providedirect two-wheel drive and unlocked to provide four-wheel drive at areduced speed (underdrive) mode.

It is a still further object of the present invention to provide atransfer case having an open center differential wherein skid control isprovided by, for example, an anti-lock braking system incorporated inthe motor vehicle which is independent of the transfer case.

It is a still further object of the present invention to provide a motorvehicle transfer case wherein an open center differential and unequalchain sprockets provide drive torque to primary and secondary drivelines in a four-wheel underdrive mode to improve vehicle performance.

Further objects and advantages of the present invention will becomeapparent by reference to the following description and appended drawingwherein like reference numbers refer to the same component, element orfeature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic, plan view of a four-wheel drive motor vehiclepowertrain having a transfer case incorporating the present invention;

FIG. 2 is a full, sectional view of a motor vehicle transfer caseincorporating a center differential according to the present invention;and

FIG. 3 is an enlarged, fragmentary sectional view of an opendifferential according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a four-wheel vehicle drive train isdiagrammatically illustrated and designated by the reference number 10.The four-wheel vehicle drive train 10 includes a prime mover 12 which iscoupled to and directly drives a transmission 14. The transmission 14may either be an automatic or manual type. The output of thetransmission 14 directly drives a transfer case assembly 16 whichprovides motive power to a primary or rear drive line 20 comprising aprimary or rear prop shaft 22, a primary or rear differential 24, a pairof live primary or rear axles 26 and a respective pair of primary orrear tire and wheel assemblies 28.

The transfer case assembly 16 also selectively provides motive power toa secondary or front drive line 30 comprising a secondary or front propshaft 32, a secondary or front differential assembly 34, a pair of livesecondary or front axles 36 and a respective pair of secondary or fronttire and wheel assemblies 38. The front tire and wheel assemblies 38 arepreferably directly coupled to a respective one of the pair of frontaxles 36. Alternately, a pair of manually or remotely activateablelocking hubs 42 may be operably disposed between the pair of front axles36 and a respective one of the tire and wheel assemblies 38 toselectively connect same. Both the primary drive line 20 and thesecondary drive line 30 may include suitable and appropriately disposeduniversal joints 44 which function in conventional fashion to allowstatic and dynamic offsets and misalignments between the various shaftsand components.

An operator control console or assembly 46 is preferably disposed withineasy reach of the vehicle operator and includes a switch or a pluralityof pushbuttons 48 which select one of four operating modes of thetransfer case assembly 16: H2—two-wheel drive by the primary drive line20 in high gear (direct drive); N—neutral; H4—four-wheel drive throughthe primary and secondary drive lines 20 and 30 in reduced high gear(underdrive) and L4-four-wheel drive through the primary and secondarydrive lines 20 and 30 in low gear.

The foregoing and following description relates to a vehicle wherein therear drive line 20 functions as the primary drive line, i.e., it isengaged and operates substantially all the time and, correspondingly,the front drive line 30 functions as the secondary drive line, i.e., itis engaged and operates only part-time or in a secondary or supplementalfashion, such a vehicle commonly being referred to as a rear wheel drivevehicle.

These designations “primary” and “secondary” are utilized herein ratherthan “front” and “rear” inasmuch as the invention herein disclosed andclaimed may be readily utilized in transmissions and transfer caseswherein the primary drive line 20 is disposed at the front of thevehicle and the secondary drive line 30 is disposed at the rear of thevehicle. Such designations “primary” and “secondary” thus broadly andproperly characterize the function of the individual drive lines ratherthan their specific locations.

Referring now to FIGS. 1 and 2, the transfer case assembly 16incorporating the present invention includes a multiple piece, typicallycast, housing assembly 50 having planar and circular sealing surfaces,openings for shafts and bearings and various recesses, shoulders,flanges, counterbores and the like to receive various components orassemblies of the transfer case 16. An input shaft 52 includes female orinternal splines or gear teeth 54 or other suitable structure whichdrivingly couple an output of the transmission 14 illustrated in FIG. 1to the input shaft 52. The input shaft 52 is rotatably supportedexternally by an anti-friction bearing such as the ball bearing assembly56 and internally by an anti-friction bearing such as the roller bearingassembly 58. The roller bearing assembly 58 is disposed upon a portionof a stepped, intermediate shaft 60. A suitable oil seal 62, positionedbetween the input shaft 52 and the housing assembly 50, provides anappropriate fluid tight seal therebetween. The opposite end of theintermediate shaft 60 is supported by an anti-friction bearing such as aroller bearing assembly 64. An end cap or seal 66 closes off the end ofan axial passageway 68 in the intermediate shaft 60. A gerotor pump Pillustrated in phantom lines will typically be utilized to provide aflow of lubricating and cooling fluid to the axial passageway 68 whichis thence distributed through a plurality of radial ports in theintermediate shaft 60 to the components of the transfer case assembly16.

Referring now to FIG. 2, the transfer case assembly 16 includes atwo-speed planetary (epicyclic) gear assembly 70 disposed generallyabout the input shaft 52. The planetary gear assembly 70 includes a sungear 72 having a plurality of external gear teeth 74 and a plurality ofinternal splines or gear teeth 76 which are both formed on an axialextension 78 of the input shaft 52. Radially aligned with the sun gear72 and its teeth 74 is a ring gear 80 having internal gear teeth 82. Thering gear 80 is fixedly retained within the housing assembly 50 by anysuitable retaining structure such as a projection or lip 84 formed inportions of the housing assembly 50 and a cooperating snap ring 86. Aplurality of pinion gears 88 are rotatably received upon a likeplurality of anti-friction bearings such as roller bearings 90 which, inturn, are supported and located by a like plurality of stub shafts 92.The plurality of stub shafts 92 are mounted within and secured to aplanet carrier 94. The planet carrier 94 includes a plurality ofinternal splines or gear teeth 96 disposed generally adjacent theinternal splines or gear teeth 76 on the extension 78 of the input shaft52. The planetary gear assembly 70 is more fully described in co-ownedU.S. Pat. No. 4,440,042 which is herein incorporated by reference.

The planetary gear assembly 70 also includes a first dog clutch orclutch collar 100 defining elongate internal splines or gear teeth 102.The internal splines or gear teeth 102 of the first clutch collar 100are slidably received upon a complementary plurality of external splinesor gear teeth 104 on the intermediate shaft 60. The first clutch collar100 thus rotates with the intermediate shaft 60 but may translatebi-directionally therealong. The first clutch collar 100 also includesexternal splines or gear teeth 106 on one end which are in all respectscomplementary to the internal splines or gear teeth 76 on the axialextension 78 of the input shaft 52 and the internal splines or gearteeth 96 on the planet carrier 94. The opposite end of the first clutchcollar 100 defines a circumferentially and radially extending flange108.

The first clutch collar 100 is capable of three positions andoperational modes. In the lower portion of FIG. 2, the first clutchcollar 100 is illustrated in its leftmost or direct drive position.Direct drive is achieved when the external splines or gear teeth 106 ofthe first clutch collar 100 engage the internal splines or gear teeth 76on the axial extension 78 of the input shaft 52 thereby directlycoupling the input shaft 52 to the intermediate shaft 60 and providingdirect or high gear drive therebetween. When the first clutch collar 100is moved to the right from the position illustrated in the lower portionof FIG. 2, to the position illustrated in the upper portion of FIG. 2,the speed reduction achieved by the planetary gear assembly 70 isengaged through engagement of the external splines or gear teeth 106 onthe first clutch collar 100 with the internal splines or gear teeth 96on the planet carrier 94. So engaged, the planetary gear assembly 70 isactive and provides a speed reduction, typically in the range of from3:1 to 4:1 between the input shaft 52 and the intermediate shaft 60.Between these two positions is a neutral position. In the center,neutral position both the input shaft 52 and the planet carrier 94 aredisconnected from the intermediate shaft 60 and no power is transmittedtherebetween.

The position of the first clutch collar 100 is commanded by an electricshift control motor 110. The shift control motor 110 rotates a driveshaft or shift rail 112 which is supported for rotation in the housingassembly 50 by a pair of bushings or bearings 114. The shift rail 112includes a first pair of radially extending, spaced apart pins or camfollowers 116. Disposed between the first pair of cam followers 116 is afirst shift fork assembly 120. The first shift fork assembly 120includes a first shift fork body 122 defining a through passageway 124which receives the shift rail 112. At opposite ends of the first shiftfork body 122 are complementarily configured, spaced apart helical camsurfaces 126 and axially extending discontinuities or flats 128. Thehelical cam surfaces 126 are configured and spaced a distance slightlyless than the separation of the cam followers 116 such that rotation ofthe shift rail 112 axially translates the shift fork assembly 120 alimited distance along the shift rail 112. The discontinuities or flats128 function as stops which limit rotation of the shift rail 112 in onedirection. The shift fork assembly 120 also includes a yoke 132 whichdefines a semicircular channel or groove 134. The semicircular groove134 is complementary to and engages a portion of the flange 108 on thefirst clutch collar 100 thus axially and bidirectionally repositioningthe first clutch collar 100 in response to rotation of the shift rail112, as will be readily appreciated.

It should be understood that the planetary gear assembly 70 includingthe first clutch collar 100 which provides dual range, i.e., high andlow speed, capability of the transfer case assembly 16 is optional, thatthe four-wheel vehicle drive train 10 and the transfer case assembly 16are fully functional as a single speed, direct drive unit and that thepresent invention may be utilized without the planetary gear assembly 70and the dual speed range capability provided thereby.

Turning now to FIGS. 2 and 3, the transfer case assembly 16 alsoincludes a planetary gear, center differential assembly 140. The centerdifferential assembly 140 is preferably also an epicyclic gear traindevice and includes a planet carrier 142 having internal splines or gearteeth 144 which engage and are driven by complementarily configuredexternal splines or gear teeth 146 formed on a stepped, terminal portion148 of the intermediate shaft 60. The planet carrier 142 receives aplurality, preferably three, axially extending stub shafts 150 whichsupport needle bearing assemblies 152 which rotatably receive aplurality of pinion or planet gears 154. The plurality of pinion orplanet gears 154 each include gear teeth 156 which are in constant meshwith internal gear teeth 158 formed on the interior of a ring gearannulus 162. The ring gear annulus 162 is engaged by, coupled to andsupported by a plurality of external gear teeth 164 formed on theperiphery of an obliquely extending circular member 166 whichconstitutes a portion of a primary output shaft 168. A snap ring 170seated in a circumferential groove in the gear teeth 158 retains thecircular member 166 of the output shaft 168 within the ring gear annulus162.

As noted above, the roller bearing assembly 64 rotatably supports thestepped, terminal portion 148 of the intermediate shaft 60 in acounterbore in the primary output shaft 168. The primary output shaft168 is preferably rotatably-supported upon an anti-friction bearing suchas a ball bearing assembly 172. The primary output shaft 168 may includeexternal splines or gear teeth 174 which mate with internal splines orgear teeth 176 in an output flange 178 or other output or drivelinestructure. The output flange 178 is preferably secured to the primaryoutput shaft 168 by a suitable retainer such as a nut 182. An oil seal184 provides a fluid tight seal between the flange 178 and the housing50 of the transfer case assembly 16.

At the opposite end of the ring gear annulus 162 from the obliquelyextending circular member 166 is a circular end plate 192. The circularend plate 192 includes external gear teeth 194 which are complementaryto and engage the internal gear teeth 158 on the ring gear annulus 162.The circular end plate 192 is disposed adjacent the planet carrier 142and is axially restrained by a snap ring 196 received within a suitablecircumferential groove in the gear teeth 158. The circular end plate 192also includes a plurality of internal splines or gear teeth 198. It willbe appreciated that various friction reducing flat washers 200 may bedisposed between components of the center differential assembly 140which rotate at different speeds.

A drive sleeve 202 is freely rotatably disposed about the intermediateshaft 60. A friction reducing bushing 204 may be disposed between thedrive sleeve 202 and the intermediate shaft 60, if desired. The drivesleeve 202 includes a plurality of external gear teeth 206 which are inconstant mesh with the gear teeth 156 of each of the plurality of pinionor planet gears 154. The drive sleeve 202 and its external gear teeth206 thus function as a sun gear, cooperating with the pinion or planetgears 154 which, in turn, cooperate with the gear teeth 158 of the ringgear annulus 162 to form a planetary or epicyclic gear train which, asarranged, functions as an open center differential.

The drive sleeve 202 also includes a region of external splines or gearteeth 208 which are disposed generally opposite the external gear teeth206 which form the sun gear. The drive sleeve 202 is retained andaxially positioned by a snap ring 212 which is received within asuitable circumferential groove 214 formed in the intermediate shaft 60.Axially, bi-directionally slidable upon the external splines or gearteeth 208 of the drive sleeve 202 is a second dog clutch or clutchcollar 220. The second clutch collar 220 includes internal splines orgear teeth 222 which are complementary to and drivingly engaged by theexternal splines or gear teeth 208 on the drive sleeve 202. The end ofthe second clutch collar 220 more proximate the center differentialassembly 140 includes a first set of external splines or gear teeth 224which are in all respects complementary to and engageable with theinternal splines or gear teeth 198 on the circular end plate 192.

The second clutch collar 220 includes a medially disposed, radially andcircumferentially extending flange 226. The second clutch collar 220also includes a second set of external splines or gear teeth 228disposed at the end of the second clutch collar 220 opposite the firstset of external splines or gear teeth 224.

As illustrated in FIG. 2, a second shift fork assembly 230 is alsodisposed upon the shift rail 112. A second pair of pins or cam followers232 engage spaced apart helical cams 234 on opposite ends of a secondshift fork body 236. The helical cams 234 may include axially extendingdiscontinuities or flats 238. The second shift fork body 236 alsoincludes a through passageway 240 adapted to receive the shift rail 112.The second shift fork assembly 230 also includes a yoke 242 defining asemi circular groove 244 which receives a portion of the flange 226 ofthe second clutch collar 220.

Returning on FIGS. 2 and 3, a smaller chain drive sprocket 250 is freelyrotatably disposed upon the intermediate shaft 60. The smaller chaindrive sprocket 250 is located and axially restrained between a shoulder252 formed on the intermediate shaft 60 and a snap ring 254 which isseated within a complementarily configured circumferential groove 256.The smaller chain drive sprocket 250 include internal splines or gearteeth 258 which are complementary to and may be selectively engaged bythe second plurality of external splines or gear teeth 228 on the secondshift collar 220. The smaller chain drive sprocket 250 also includes aplurality of chain drive teeth 262 which drivingly engage a drive chain264. The drive chain 264, in turn, drivingly engages chain teeth 266formed about the periphery of a larger driven chain sprocket 268. Thelarger driven chain sprocket 268 preferably includes internal splines orgear teeth 272 which are complementary to and engage external splines orgear teeth 274 on a secondary output shaft 276. The secondary outputshaft 276 is preferably supported upon a pair of spaced apartanti-friction bearings such as the ball bearing assemblies 282. Thesecondary output shaft 276 includes a flange 278 or other structurecompatible with the related, driven components of the secondary driveline 30. An oil seal 284 provides a suitable fluid tight seal betweenthe secondary output shaft 276 and the housing 50 of the transfer caseassembly 16.

The operation of the transfer case assembly 16 and specifically thecenter differential assembly 140 in the two-wheel drive, high gear(direct drive) mode and the four-wheel drive, reduced speed (underdrive)mode according to the present invention will now be described. In FIG.3, the second clutch collar 220 is illustrated in the two-wheel, directdrive operating mode. In this operating mode, the second clutch collar220 is in its right-most position with the external splines or gearteeth 224 of the second clutch collar 220 engaged with the internalsplines or gear teeth 198 on the circular end plate 192. Since thesecond clutch collar 220, through the drive sleeve 202 is rotationallyconnected to the sun gear teeth 206 and thence to the pinions or planetgears 154, the center differential assembly 140 is effectively lockedand drive torque entering the center differential assembly 140 throughthe carrier 142 is provided to the primary output shaft 168 withoutdifferentiation, speed increase or speed reduction.

In the four-wheel, underdrive operating mode, the second shift collar220 is translated by the shift control motor 110 to the left-mostposition illustrated in phantom lines in FIG. 3. Drive torque isintroduced into the center differential assembly 140 from theintermediate shaft 60, through the carrier 142 and the associated pinionor planet gears 154. The second shift collar 220 is engaged with anddrives the smaller chain drive sprocket 250. The center differentialassembly 140 is thus unlocked and may achieve differentiation betweenthe primary output shaft 168 and the smaller chain drive sprocket 250.Preferably, the center differential assembly 140 is configured toprovide a 65/35 torque split between the primary drive line 20 and thesecondary drive line 30.

In this operating mode, speed reduction is achieved through the centerdifferential assembly 140 to the primary output shaft 168 relative tothe direct drive operating mode discussed above. A corresponding speedreduction is achieved through the ratio of the gear teeth on the smallerchain drive sprocket 250 and the larger driven chain sprocket 268. Forexample, the smaller chain drive sprocket 250 may include thirty-oneteeth whereas the larger driven chain sprocket 268 may include forty-oneteeth thereby achieving a speed reduction ratio of 1.1323 to 1 which,when combined with the ratio between the pinions or planet gears 154 andthe sun gear teeth 206 on the drive sleeve 202 achieves the same nominalreduction as that through the pinions or planet gears 154, the ring gearannulus 162 and the primary output shaft 168. Reduction ratios betweenthe smaller chain drive sprocket 250 and the larger driven chainsprocket 268 in the range of 1.15 to 1.50 to 1.00 are suitable.

While this ratio may be varied to accommodate various vehicle sizes,weights and horsepowers, the overall operating reduction ratio in theunderdrive mode of 1.13 to 1 has been found satisfactory. Ratios in therange of about 1.05 to 1 to 1.25 to 1 are also considered suitable. Thisreduction (underdrive) ratio effectively increases the axle ratios ofthe front and rear differentials from, for example, 3.31 to 1 to 3.73 to1, thereby improving the performance and towing capability of thevehicle. Of course, when the vehicle is shifted back to two-wheel,direct drive, it performs in accordance with its standard, for example,3.31 to 1 axle ratio as will be readily appreciated.

As noted above, the center differential assembly 140 providing anunderdrive speed range in high gear may be utilized with or without thetwo speed planetary gear assembly 70 and the high (direct) and low(reduced) speed ranges it provides. Furthermore, the center differentialassembly 140 according to the present invention in conjunction with theprimary differential 24 and the secondary differential 34 providecomplete differentiation, i.e., permit independent variable speeds amongthe tire and wheel assemblies 28 and 38. Accordingly, the antilock orantiskid brake control system of the associated motor vehicle may beutilized to control and limit torque distribution to the various tireand wheel assemblies 28 and 38 in order to reduce wheel spin and improvevehicle performance and handling, particularly in adverse weather anddriving conditions.

The foregoing disclosure is the best mode devised by the inventor forpracticing this invention. It is apparent, however, that apparatusincorporating modifications and variations will be obvious to oneskilled in the art of transfer cases and motor vehicle drive lines.Inasmuch as the foregoing disclosure presents the best mode contemplatedby the inventor for carrying out the invention and is intended to enableany person skilled in the pertinent art to practice this invention, itshould not be construed to be limited thereby but should be construed toinclude such aforementioned obvious variations and be limited only bythe spirit and scope of the following claims.

I claim:
 1. A transfer case for a four-wheel drive vehicles comprising,in combination, an input, a first output for driving a first drive line,a second output for driving a second drive line, said second outputincluding first and second chain sprockets having disparate numbers ofteeth and a chain engaging said chain sprockets, a planetary centerdifferential having a carrier driven by said input, a plurality ofplanet gears rotatably disposed in said carrier, a ring gear engaging bysaid planet gears and driving said first output and an output sleevehaving teeth engaging said planet gears, and a clutch collarrotationally coupled to said output sleeve and translatable to a firstposition operably engaging said ring gear and a second position operablyengaging said second output.
 2. The transfer case of claim 1 wherebysaid first position of said clutch collar locks said center differentialand said second position provides an underdrive operating mode.
 3. Thetransfer case of claim 1 wherein said chain sprocket engaged by saidclutch collar includes fewer teeth than said other chain sprocket. 4.The transfer case of claim 1 further including a speed reductionassembly having a first, direct drive output, a second, reduced speedoutput and a clutch for selectively engaging one of said outputs to saidinput.
 5. The transfer case of claim 1 further including a shift rail, ashift fork received on said shift rail and engaging said clutch collarand a drive motor for moving said clutch collar.
 6. The transfer case ofclaim 1 wherein said first position of said clutch collar providesdirect drive and said second position of said clutch collar provides areduced speed drive.
 7. A transfer case for a four-wheel drive vehiclecomprising, in combination, an input for receiving drive energy, a firstoutput for driving a first drive line, a second output for driving asecond drive line, said second output including first and second chainsprockets having disparate numbers of teeth and a chain engaging saidchair sprockets, a planetary center differential having a carrieroperably coupled to said input, a plurality of planet gears rotatablydisposed in said carrier, a ring gear engaging said planet gears andoperably coupled to said first output and an output sleeve having teethengaging said planet gears, and a clutch collar rotationally coupled tosaid output sleeve and translatable no a first position operablyengaging said ring gear and a second position operably engaging saidsecond output.
 8. The transfer case of claim 7 whereby said firstposition of said clutch collar provides direct, two-wheel drive and saidsecond position of said clutch collar provides four-wheel driveunderdrive.
 9. The transfer case of claim 7 wherein said chain sprocketengaged by said clutch collar includes fewer teeth than said other chainsprocket.
 10. The transfer case of claim 9 wherein said chain sprocketengaged by said clutch collar includes 31 teeth and said other chainsprocket includes 41 teeth.
 11. The transfer case of claim 7 furtherincluding a speed reduction assembly having a first, direct driveoutput, a second, reduced speed output and a clutch for selectivelyengaging one of said outputs to said input.
 12. The transfer case ofclaim 7 wherein said first position of said clutch collar providesdirect drive and said second position of said clutch collar provides areduced speed drive.
 13. A transfer case for a four-wheel drive vehiclecomprising, in combination, an input, a first output for driving a firstdrive line, a second output for driving a second drive line, said secondoutput including a speed reducing assembly, said speed reducing assemblyincludes a first chain sprocket having a first number of teeth and asecond chain sprocket having a second number of teeth greater than saidfirst number of teeth, a planetary differential having a carrier drivenby said input, a plurality of planet gears rotatably disposed in saidcarrier, a ring gear engaging by said planet gears and driving saidfirst output and an output sleeve having teeth engaging said planetgears, and a clutch member driven by said output sleeve and translatableto a first position operably engaging said ring gear and a secondposition operably engaging said second output, whereby said firstposition of said clutch member provides direct, two-wheel drive and saidsecond position of said clutch member provides four-wheel driveunderdrive.
 14. The transfer case of claim 13 wherein said first numberof teeth is 31 and said second number of teeth is
 41. 15. The transfercase of claim 13 wherein said underdrive ratio is between about 1.05 to1.25 to
 1. 16. The transfer case of claim 13 further including aplanetary speed reduction assembly having a first, direct drive output,a second, reduced speed output and a clutch for selectively engaging oneof said outputs to said input member.
 17. The transfer case of claim 13further including a shift rail, a shift fork received on said shift railand engaging said clutch member and a drive motor for moving said clutchmember.